1. Field
The invention relates to a method of producing a photo-oriented structurally anisotropic polymer network ("PPN") having any desired azimuthal orientation direction and a tilt angle on its surface, to orientation layers prepared by this method for orienting monomeric and polymeric liquid crystals, to the use of this method, and the orientation layers produced thereby.
2. Description
Uniaxially rubbed polymer orientation layers, such as polyimide are conventionally used to orient liquid crystal molecules in liquid crystal displays ("LCD"). The rubbing direction determines the orientation direction and during the rubbing process a tilt angle is produced on the polymer surface.
When a liquid crystal is placed in contact with such a surface the liquid crystal molecules are disposed not parallel to the surface but at an inclination thereto--that is, the tilt angle is transferred to the liquid crystal. The size of the tilt angle is determined by rubbing parameters, for example, feed rate and pressure, and by the chemical structure of the polymer. For example, there are many structurally different polyimides which when subjected to identical preparation and rubbing parameters lead to completely different tilt angles. Tilt angles between 1.degree. and 15.degree. depending upon type, are necessary for the preparation of liquid crystal displays. The larger tilt angles are required more particularly for supertwisted nematic ("STN") LCDs in order to avoid the incidence of so-called finger print textures. In twisted nematic ("TN") and thin film transistor ("TFT")-TN-LCDs, the tilt angle defines the rotational and the tilt direction so that reverse twist and reverse tilt phenomena are inhibited. Reverse twist in the "off" state leads to zones with a wrong direction of twist, a phenomenon which is optically perceptible as a spotty appearance of the display. In contrast, reverse tilt (occuring more particularly when the LCDs are actuated) causes a very disturbing optical effect due to the liquid crystals tilting in different directions. Also, reverse twist can be inhibited by doping the liquid crystal mixture with a chiral dope of appropriate direction of rotation. However, to suppress reverse tilt there is no alternative but to use orientation layers having a tilt angle.
Although rubbed polymer layers have proved very satisfactory for orienting liquid crystals in LCD production, there are a number of serious disadvantages causally related to the rubbing. Because of optically inadequate displays, LCD production yield is unsatisfactory since rubbing (1) is associated with the production of dust and (2) produces an electrostatic charge on the polymer layer. This can result, for example in the case of TFF-TN LCDs, in the destruction of thin film transistors below, as well as the attraction of additional dust to the surface. Another serious disadvantage is that when a large area is rubbed, the orientation direction cannot be varied locally. Accordingly, there is therefore no practical way of enhancing the viewing angle dependency of TN LCDs.
Recently orientation layers have become known wherein the orientation direction can be determined by exposure with linearly polarized light. The problems inherent in rubbing can therefore be avoided. The additional possibility of zone-wise differentiation of orientation directions opens up completely new possibilities for optimising the properties of liquid crystal displays, for example, the viewing angle dependency of TN LCDs.
U.S. Pat. No. 4,974,941, the contents of which are herein incorporated by reference, describes a process based on a guest-host system wherein a preferred direction is induced in response to exposure with linearly polarized light of an appropriate wavelength by the cis/trans-isomerization of dyes. Liquid crystals in contact with a surface thus exposed are oriented in accordance with this preferred direction. This orientation process is reversible--that is, by further exposure of the layer to light of a second polarization direction the orientation direction already written in can be rotated again. Since this re-orientation process can be repeated as often as required and needs high light intensities, orientation layers on this basis are less suitable for use in LCDs.
In contrast to this reversible orientation process, in the photostructurable orientation layers such as described in U.S. Pat. No. 5,389,698, the contents of which is herein incorporated by reference, an irreversible anisotropic polymer network is built up. The anisotropic orienting properties induced in the network during the exposure to linearly polarized light are photostable and so cannot be further re-oriented by further exposures. The photo-orientated polymer networks ("PPN") are therefore of use more particularly wherever stable, structured or unstructured liquid crystal orientation layers are required. In addition to being used in LCDs, orientation layers of this kind are useful in preparating other optical elements, such as non-absorptive color filters, linear and cholesteric polarizing filters, optical delay layers, and the like.
More particularly for use in LCDs, the orientation layer must transfer the tilt angle as well as the orientation direction. However, endeavors to induce a tilt angle in photostructurable orientation layers have so far proved unsatisfactory. The only process known so far has been described in Hashimoto, T. et al in SID 95 DIGEST, 877 (1995) wherein a combination of two consecutive exposures in different conditions can produce a tilt angle. The first exposure is carried out with a vertical incidence of light whereas in the second exposure the incident of light is grazing, the direction of polarization of the light having to be turned through 90.degree. relative to the first exposure. The orientation direction of the polyvinyl cinnamate photopolymer used is perpendicular to the polarization direction of the incident UV light. Consequently, only the orientation direction is determined in the first exposure and for reasons of symmetry no preferred direction for the tilt angle can be given. In the second exposure with inclined incidence of light and a 90.degree. offset polarization direction an orientation is built up perpendicularly to the previous orientation, something which of course reduces the orientability of the first direction. A tilt angle arises because of the asymmetrical decrease of the orientation produced by the first exposure. A compromise must therefore be made in the second exposure--the second exposure time must be long enough to induce a tilt angle, but short enough not to completely destroy the existing orientation. The PPN material must necessarily still not be photostable after termination of the first exposure if it is to be possible to make the second exposure. Consequently, and because of unsatisfactory thermal stability, the polyvinyl cinnamate used (which is of course not photostable) is of little use for LCDs.
In the known PPN materials and PPN orientation processes, irradiation with linearly polarized UV light incident parallel to the normal to the surface is used to produce an orientation perpendicular to the polarization direction of the light. The PPN materials having this property will be called hereinafter "perpendicularly orienting", whereas PPN materials which under the same conditions produce an orientation parallel to the polarization direction of the UV light will be called "parallelly orienting".
It is the object of the invention to devise a simple process, and appropriate photostructurable materials, such that during exposure a defined long-term-stable tilt angle combined with any required azimuthal angle can be programmed.